I set my wife up with a DA11-30 cassette for Nice and she ran it with the 9070 derailleur no problem.

Technically, Shimano says no: https://productinfo.shimano.com/...C-453&acid=C-454

But they are hyper conservative. 2 more teeth should not be a problem. Just start with a new chain cut to length to ensure you can still get to the big-big combo without breaking things.

Thanks guys...

I tried with my 9070 derailleur but couldn't. You need a longer cage as the top pulley teeth in the cage start running against the cassette as you get to your 30.

Which derailleur did you wind up getting??

I ran a 28 not the 30 which is still sitting brand new in my garage... I am building a new bike with a 9150 derailleur which has a cage long enough to use it. It was a one off 70.3 Worlds and the 30 would have been nice on that course but I just suffered with the 28.

You may be able to put a longer cage on the derailleur or upgrade to the 9150 or the Ultegra equivalent.

I think it depends on the geometry of your mech hanger what you can get away with.
That’s why some are okay with a 30, yet others aren’t.

Did you adjust the B screw? Maybe try the trick with putting the B screw in in reverse?

I'm running a Di2 9070 RD using an Ultegra 11-30, albeit in a 50T 1x setup, which takes chain length problems out of the equation. This is without the B screw put in in reverse - I might be able to get away with an 11-32 if I do that.

As alluded to above, frame hanger geometry can play into all of this.

As for what others are saying maybe you can on some bikes but at maximum adjustment the top wheel teeth would touch on the cassette just as Shimano indicated it would. Maybe some bikes hanger derailleur bolt hole is lower than others giving the extra few mm required for clearance?

The frame most likely impacts things a bit. But if you put the B screw in from the other side you will also gain a bit (the length of the screw head) which can do the trick in some cases.

It could also be the rear derailleur is a little further back, the chain tension higher or B screw adjustment is further in.

I run a 11-34t cassette with a DA9100 rear derailleur on two separate bikes that have 50/34 cranks and do not have any issues after adjusting the B screw.

You obviously don't have a short cage derailleur

The Dura Ace 9100 mechanical rear derailleur comes in only one size but it is a longer cage than the earlier versions of DA.